1. Field of the Invention
This invention relates to single mode optical fibers and, more particularly, to reducing bend sensitivity in such fibers. It also relates to fiber designs that are not only relatively bend insensitive but also effectively mode-matched to standard single mode fiber and/or have effectively cut off higher order transverse modes.
2. Discussion of the Related Art
In contrast with standard single mode optical fiber used, for example, in land line, undersea and metro systems, access fiber, which is typically located closer to the user, includes fiber-to the-home (FTTH), jumper cables, and FTTx fiber (e.g., fiber-to-the-curb, indoor wiring). Access fiber must not only interface in a low loss, reliable way with standard single mode fiber (SMF), which carries optical signals to the location being accessed (e.g., home, business, or other facility), but also must be relatively insensitive to the effects of bending, which is inherent in many of the access fiber applications.
More specifically, standard step index, SMF often has a core diameter of about 8-11 μm. SMF of this type illustratively complies with International Telecommunications Union-Telecommunications Sector Recommendation G.652. A typical fiber of this type, known as SMF 28, is commercially available from Corning Inc., Corning, N.Y. With diameters in this range and a core-cladding index contrast of about 0.005-0.007, such SMFs typically have an effective fundamental transverse mode-field-area (Aeff) of about 55-70 μm2 at a wavelength of about 1300 nm and about 70-90 μm2 at a wavelength of about 1550 nm. In order to efficiently couple (splice) such a SMF to an access fiber, the Aeff of the access fiber should be as close to that of the SMF as practically possible.
In addition, however, some access fiber may have to be bent, for example, around the corners of walls and in wiring conduits or ducts. In many applications the required bend radius of the access fiber may be approximately 4-15 mm, and in some cases the lower limit may reflect even tighter bends (e.g., 2-3 mm radius). When fiber is bent so severely, however, optical loss may increase to unacceptable levels. In particular, the optical loss experienced by the fundamental transverse mode of a standard SMF, which may be within system specifications when the fiber is straight (or bent only slightly), is often much higher and out of specification when the same fiber is bent too much. In addition, if the optical signal contains any higher order transverse modes (HOMs), the bend loss experienced by such HOMs will be even higher than that suffered by the fundamental transverse mode.
As nature would have it, however, the design of an access fiber that has a Aeff matched to that of a standard SMF and/or with HOMs effectively cutoff often conflicts with the design that achieves low bend loss in typical access environments.
Therefore, a need remains in the art for an access fiber design that has relatively low bend loss when bent to a radius of about 4-15 mm and simultaneously has a Aeff matched to that of standard SMF and/or has HOMs effectively cutoff.